The National Oceanic and Atmospheric Administration of America (NOAA) launched weather satellites in the 1970’s. These satellites stream down images of the planet at around 137MHz. At the moment, there are three satellites still active that can be received using only a software defined radio and a circularly polarised antenna. They are:
- NOAA 15 – 137.6 MHz
- NOAA 18 – 137.9 MHz
- NOAA 19 – 137.1 MHz
There are heaps of examples online of people building Quadrifilar Helix Antennas (QHA) to receive the NOAA transmissions. So… that’s what I decided to do!
I built the antenna using the design found in the ARRL. There are lots of different antenna designs online, some using copper for the conductor, some using coax, but all seem to work pretty well. The antenna consists of two loops, each a half turn helix, that are slightly different lengths. The different lengths means that the induced current from the satellite signal lags in one loop and leads in the other to produce a 90 degree phase difference (to receive the circularly polarised signal). You can see a picture of the antenna on its side below, and also mounted on my roof. The main difference between the ARRL design and others is that one half of the larger loops is made of coax to form an infinite balun. (You need a balun because the input to the SDR is unbalanced, whereas the circularly polarised signals are).
You can use the free program gpredict to find out when the next satellite will pass by.
Using gqrx, you can receive the satellite signals and record them as narrow FM. The signals in gqrx are shown in the next pictures.
The first picture is NOAA 18 and the second is NOAA 15. A recording of the signal that the satellites transmit sounds like this:
Once decoded, you can get some pretty cool pictures out of them:
These next pictures were taken from passes of NOAA 15 and NOAA 18. I’ve also included a picture from the Bureau of Meteorology’s website showing Australia’s cloud cover at the time (which is exactly what you can see in the images I got!)
And in this picture (IR enhanced) you can clearly see the Eastern part of Australia as well as Tasmania.
And finally there’s this gem:
To build the antenna, I followed the instructions found here: arrl_qha. I did make one minor modification after testing the design. The connections at the bottom of the antenna are quite fragile as you are soldering the centre conductor of two pieces of coax together (one for one half of the big loop, one for the output to the SDR) as well as soldering the shields together and also soldering on the other half of the big loop. When I was first mounting the antenna on a pole I accidentally broke this connection, so I made a small change to make it stronger.
What I would recommend doing is using the same piece of coax that forms one half of the big loop as the cable that is connected to the SDR (so effectively you now have one massive cable going from the top of the antenna, winding its way around the antenna as a helix, and then all the way to the SDR). For the connection at the bottom of the antenna, simply strip off the insulation on the outside of the coax to expose the shielding and solder the “copper” part of the big loop directly onto the shield. Electrically this is the same as the connection in the ARRL guide. This is shown in the drawing below.